I know the answer is probably easy, but how do you start over? I deleted my old characters and created new ones. Now Gareth no longer appears at the beginning to tell me to get the Mavin Sword. Also, the reptillian guy outside the gates no longer comes. Help!!

Under Options - Game there is a button to "Reset Adventure".
You didn't need to delete your characters - this is how I started off, adventured to go up a level or so and then restarted with better characters.

(But if you want to start a new party and keep the old quests etc, you must first save a game and return the old party to the Inn).